Our objective is an understanding of the means by which the diverse cell types of the limb bud arise from an apparently uniform population of limb mesenchyme cells. The following expriments are planned. 1) Autoradiographic analyses of the muscle-forming regions will determine if the first cells to leave the division cycle are the first cells to enter multinucleate straps. 2) Electron microscopy and autoradiography will be used to study initiation of limb outgrowth to investiate the relationships between the known changes in division rate and spreading, and extracellular and intracellular changes in ultrastructure. 3) The same methods of electron microscopy and autoradiography will be used to determine if post-mitotic cells can be detected by ultrastructural criteria. 4) The time of stabilization of the cells in the muscle- forming region will be determined by the method that has been successfully used for the cartilage-forming region. This involves transplantation of blocks of premuscle tissue to the precartilage region at successively later stages and the analysis of the retention of muscle properties by electron microscopy. 5) The synthesis of actin and myosin will be studied by incorporation of radioactive amino acids into proteins with the solubility and electrophoretic mobility of actin and myosin. 6) The effects of teratogenic agents specific for the destruction of either muscle or cartilage cells will be investigated by the same techniques as have been used in the study of normal development.